1. Field of the Invention
The present invention relates to a third overtone crystal oscillator, and more particularly, to a third overtone crystal oscillator which is configured by using an IC (integrated circuit) for fundamental wave oscillation of a crystal element.
2. Description of the Related Arts
Crystal oscillators configured by combining a quartz crystal element and an integrated circuit, which is provided with oscillation circuit employing the crystal element, have excellent frequency stability. The crystal oscillator are therefore used as a reference source for frequency and time in various kinds of electronic devices. One type of IC used for such crystal oscillators includes, for example, ICs which are manufactured by Seiko NPC Corporation, Tokyo, JAPAN and known by version names CF5036 series and CF5037 series. These series of ICs are used for crystal oscillators for optical digital networks.
In recent years, in order to double the transmission capacity of a network, an crystal oscillator for optical digital networks which has an oscillation output in 300-MHz band has been demanded to replace the currently used one having oscillation output in 150-MHz band.
FIG. 1A is a circuit diagram illustrating an example of a configuration of a conventional crystal oscillator, FIG. 1B is a plan view of the crystal oscillator with a cover removed therefrom, and FIG. 1C is a cross sectional view of the crystal oscillator.
The crystal oscillator includes oscillator IC 1 having an integrated oscillation circuit, and quartz crystal element (crystal blank) 2, both of which are accommodated in a recess formed in container 3. Here, it is assumed that oscillator IC 1 is one from CF5036 and CF5037 series manufactured by Seiko NPC Corporation. Such oscillator IC 1 is configured by integrating at least transistor Tr for oscillation, constant current source I, first and second capacitors C1 and C2 for oscillation, and DC (direct current) blocking capacitor Cs. Transistor Tr is grounded at the emitter thereof, with bias resistor R being connected between its collector and base.
Constant current source I is to generate a constant current by being supplied with power supply voltage Vcc, and thus supplies the constant current to a node between the collector of transistor Tr and bias resistor R. First capacitor C1 for oscillation is connected between the base of transistor Tr and the ground potential, while second capacitor C2 is connected between the collector and the ground potential. Direct-current blocking capacitor Cs is inserted between the node where the base meets with bias resistor R, and first capacitor C1. Oscillator IC 1 is provided with output terminal Vout which is connected to the collector of transistor Tr.
Container 3 is made up, for example, of laminated ceramics in which the recess is formed by stacking a frame wall layer having an opening at the center portion thereof, on bottom wall layer 3c having a substantially rectangular shape. The frame wall layer comprises of upper layer 3d, middle layer 3a and lower layer 3b. Lower layer 3b is stacked on bottom wall layer 3c, middle layer 3a is stacked on lower layer 3b, and upper layer 3d is stacked on middle layer 3a. The opening of middle layer 3a is formed smaller than the opening of upper layer 3d, and the opening of lower layer 3b is formed smaller than the opening of middle layer 3a. As a result, two levels of stepped portions are formed in the inner wall of the recess of container 3.
Oscillator IC 1 is die-bonded onto an inner bottom surface of the recess of container 3, that is, an exposed surface of bottom wall layer 3c. IC 1 is provided with a plurality of IC terminals for connection to external circuits. The IC terminals are led out by gold wires 4 for wire bonding 4 to the stepped portion formed in each of a pair of inner walls extending in the longitudinal direction of container 3. The stepped portion to which gold wires 4 are connected is the lower stepped portion among two levels of the stepped portions formed in the inner wall of the recess of container 3, the lower stepped portion thus corresponding to the upper surface of lower layer 3b. It should be appreciated that, terminals Q1 and Q2 of the IC terminals are used for establishing electrical connection with the crystal element.
Crystal element (crystal blank) 2 is an AT-cut quartz crystal blank, for example, and has excitation electrodes, not shown, on both principal surfaces. Leading electrodes extend from the excitation electrodes to the opposite side of the one end of the crystal blank. Crystal element 2 is held in the recess of container 3 by fixing, using conductive adhesive 5, the opposite sides of the one end of crystal element 2, to which the leading electrodes are extended, to the stepped portion of the inner wall in the one end portion in the longitudinal direction of container 3. The stepped portion to which crystal blank 2 is fixed corresponds to the upper stepped portion among two levels of the stepped portions formed in the recess of container 3, and is divided into two sections in a horizontal plane. Crystal holding terminals 7 used for electrical and mechanical connection with crystal blank 2 are formed on respective upper surfaces of these divided sections, with conductive adhesive 5 being coated on crystal connecting terminals 7. Crystal blank 2 is electrically connected between non-grounded ends of first and second capacitors C1, C2 via a pair of IC terminals Q1, Q2 of oscillator IC 1.
Oscillator IC 1 is secured to the bottom surface of container 3, followed by wire bonding mentioned above. Then, crystal blank 2 is secured to container 3 using conductive adhesive 5. After that, metal cover 8 is joined to an end face of the opening of the recess of container 3 to close the recess. Thus, oscillator IC 1 and crystal blank 2 are hermetically sealed in the recess to complete the crystal oscillator.
This crystal oscillator is able to change the operating frequency range of the oscillation circuit integrated in oscillator IC 1 by changing the circuit constants of the oscillation circuit. As the entire series of ICs, the available coverage of the operating frequency is approximately 50 to 700 MHz. Accordingly, a crystal oscillator having an oscillation frequency within a frequency range from 50 to 700 MHz can be obtained by electrically connecting crystal blank 2 having the vibrating frequency within this range to oscillator IC 1 (i.e., oscillation circuit).
When an IC in the above CF5036 and CF5037 series manufactured by Seiko NPC Corporation is used as oscillator IC 1, an oscillation output up to 700 MHz can be obtained in the case where the crystal element operates at the fundamental wave vibration mode. However, in the case of the third overtone oscillation, the oscillation frequency is limited up to 250 MHz. Thus, it has been impossible to obtain an oscillation output of a 300-MHz band with the third overtone oscillation.
In order to obtain an oscillation frequency in the 300-MHz band, crystal element 2 may only be operated with the fundamental wave vibration mode in the 300-MHz band in accordance with the specification of oscillator IC. However, the vibration frequency of AT-cut crystal element (crystal blank) 2 is in inverse proportion to its thickness, which means that a crystal element having a vibration frequency of 300 MHz at the fundamental wave vibration mode will have a thickness of about 5.6 μm. It is difficult to manufacture such a thin crystal element with a high yield. In this regard, if the oscillation frequency of 300 MHz can be obtained with the third overtone oscillation, a crystal element having a vibration frequency on the order of 100 MHz at the fundamental wave vibration mode may be used. Such a crystal element has a thickness of about 17 μm, can be readily manufactured, and can ensure stable supply.